Nichrome-chromia coating



United States Patent Ofi 3,279,939 Patented Oct. 18, 1966 3,279,939 NICHRGME-CHROMIA COATING Donald M. Yenni, Indianapolis, 11111., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Feb. 26, 1963, Ser. No. 261,191 Claims. (Cl. 117-931) This invention relates to metallic coatings, and more particularly to such coatings having high wear resistance at high temperatures.

There is an ever increasing need for such coatings. In certain applications such wear resistant materials must also be capable of forming leak tight seals. In the aircraft industry, for example, leak tight seals are needed in the high temperature environments of jet engines.

A novel self-mating, high temperature wear resistant coating has now been developed. The coating is'formed from a mixture of 215% Nichrome by weight, the balance being chromia. In the preferred form of the invention, the coating consists of 5% Nichrome and 95% chromia. The term Nichrome here means 80% nickel and 20% chromium.

The preferred method for applying the coating is the arc-torch powder coating process described in US. Patent No. 3,016,447. Such process will yield a coating that is highly dense with a lamellar, overlapping leaf structure. The coatings may be applied to either metals or non-metals. For example, the coatings could be applied to steel, aluminum, graphite, or a reinforced resin. In using this process, it is preferred to feed liquid CO to the coating area in sufficient amounts to maintain the coating area between 90-150 F. This prevents the cracking of the coating. Other similar coatings could also be used.

As previously stated, the amount of Nichrome should be between 2l5% by weight with 5% being preferred. Pure chromia will yield a good, hard, wear resistant coating for temperatures up to about 1000 F. At about 1000" F. a pure chromia coating begins to crack. At about 1400 F. the coating spalls. The addition of Nichrome serves to overcome these objections by imparting some ductility to the coating. However, the addition of too much Nichrome impairs the quality of the coating in that the coating rather easily galls. This destroys the selfmating feature of the coating.

The coating should be at least thick enough to ensure proper coverage of the base material. However, if the coating becomes too thick cracking from internal stresses arises. A thickness of from .005 inch to .015 inch has been found to be suitable.

Prior to passing the powder through the torch, the chromia and the Nichrome are ground to a particle size of no greater than -325 mesh and then mixed together. If the particle size of the chromia becomes coarser than 325 mesh, the coating has a tendency to become pitted. If the Nichrome becomes coarser than 325 mesh, there is a tendency for the coating to be easily galled.

The following examples illustrate the novel coating. In the examples apparatus of the general type depicted in the previously mentioned US. Patent No. 3,016,447 was used. The torch had a /s inch diameter thoriated tungsten electrode and a copper nozzle anode having a A; inch throat diameter.

EXAMPLE 1 In this example, 85% by weight of Cr O was mixed with by weight of Nichrome after each was ground to a -325 mesh. This powder was then passed through the torch at the rate of 5060 -gm./min. Argon gas was fed through the torch. In addition the eflluent emanating from the torch was shielded with nitrogen. Sufiicient CO (about 6 l b/min.) was passed around the torch to the area being coated to maintain the temperature of the area between and 160 F. The torch current was 200 amperes at 60 volts. The eflluent emanating from the torch was directed at a steel base 4 inch from the torch to yield a coating .010 inch thick.

EXAMPLE 2 In this example, the coating powder consisted of Cr O and 5% Nichrome by weight, preground to 325 mesh. The powder was fed at the rate of 55 gm./rnin. Argon gas Was fed through the torch. CO was used to cool the coating area in the same manner as in Example 1. The torch operated at amperes as from 55-60 volts. The efiluent emanating from the torch was directed at a graphite ring 1%. inches wide rotating at 2000 r.p.m., inch from the torch. The resulting coating was .010 inch thick.

Coatings made in the manner of these two examples have been tested and found to have a hardness of from 8001000 VPN, and to be self-mating.

What is claimed is:

1. Metallic coating having high wear resistance at high temperatures for a substrate surface consisting of 2-15% by weight of a mixture of 80% nickel and 20% chromium and the remainder chromia.

2. Metallic coating as claimed in claim 1, in which the mixture of 80% nickel and 20% chromium is of the order of 5% by weight.

3. Metallic coating as claimed in claim 1, in which the thickness of the coating is from .005 inch to .015 inch.

4. Metallic coating as claimed in claim 1, in which the coating is highly dense with a lamellar overlapping leaf structure and a hardness of from 800-1000 VPN.

5. An article of manufacture consisting of a steel base having a coating consisting of about 15% by weight of 80% nickel, 20% chromium and the balance chromia.

6. An article of manufacture consisting of a graphite base having a coating consisting of about 15% by weight of 80% nickel, 20% chromium and the balance chromia.

7. An article of manufacture comprising an aluminum base having a metallic coating as claimed in claim 1.

8. An article of manufacture comprising a reinforced resin base having a metallic coating as claimed in claim 1.

9. Arc torch powder coating process which comprises maintaining an are between a non-consumable electrode and a second electrode, passing a stream of gas to contain said are, passing said arc containing gas stream through a constricted orifice which produces a high thermal content efiluent, passing powder consisting of 215% of a mixture of 80% nickel and 20% chromium and the remainder chromia through and with said efiiuent to produce a high velocity powder laden stream, and impinging said powder laden stream against the surface of a suitable base.

10. Arc torch powder laden process as claimed in claim 9, in which liquid CO is fed to the coating area in sufficient amounts to maintain the coating area between 90- 150 F.

No references cited.

ALFRED L. LEAVITT, Primary Examiner. RICHARD D. NEVIUS, A. GOLIAN, Examiners. 

1. METALLIC COATING HAVING HIGH WEAR REISTANCE AT HIGH TEMPERATURES FOR A SUBSTRATE SURFACE CONSITING OF 2-15% BY WEIGHT OF A MIXTURE OF 80% NICKEL AND 20% CHROMIUM AND THE REMAINDER CHROMIA. 